In recent years, from the viewpoint of concern about earth environment, there is an increasing need for energy saving, and, in forming images in the electrophotography, it is required to make electric power less consumed in the fixing step that takes the considerable part of service electric power for a copying machine. For the achievement of energy saving in the fixing step, it is necessary to make toners fixable at a lower temperature. As a means for making toners fixable at a lower temperature, a technique is commonly known in which binder resins used in the toners are made to have a lower glass transition temperature. However, binder resins having too low glass transition temperature tend to cause aggregation between toner particles (a blocking phenomenon) to make it difficult to concurrently achieve storage stability of the toners.
As a means for resolving such a problem, what is called a core-shell toner is proposed in which particles serving as cores (hereinafter termed “core particles”) composed of a binder resin having a low glass transition temperature are formed and shell layers are provided as coat layers on the surfaces of the core particles.
In Japanese Patent Applications Laid-open No. 2002-116574 and No. H10-73955, a method is proposed in which core particles are previously prepared by an emulsion agglomeration process or the like and shell layers are afterwards formed thereon. In Japanese Patent Application Laid-open No. 2004-004506, a method is also proposed in which a binder resin making up core particles and an organic phase containing a colorant are dispersed in an aqueous medium in the form of droplets and thereafter a monomer making up shell layers is allowed to react at interfaces of the droplets to form shell layers thereon by interfacial polymerization.
The above emulsion agglomeration is advantageous to the controlling of internal structure of toner particles, to the controlling of content of a colorant or a release agent, to the controlling of toner particle shapes that is intentionally made and to the production of toners made to have small particle diameter, in view of the principle of granulation that agglomerates are formed on from fine particles of a dispersion of a binder resin, a colorant and a release agent each.
In the case when the core-shell toner is produced by such emulsion agglomeration, first a dispersion of a binder resin used for cores and a dispersion of a colorant are mixed and thereafter the mixture is made to agglomerate by heating, pH control and/or addition of an agglomerating agent until particles come to have the desired particle diameter, to form core agglomerated particles. Thereafter, a dispersion of a binder resin newly used for shell layers is supplementally added to form shell layers with which the core agglomerated particles are covered, to obtain core-shell agglomerated particles. Further, the core-shell agglomerated particles obtained are heated to a temperature not lower than the glass transition temperature of the binder resin to effect fusion to produce the toner.
In such a conventional method, it may come about that any fine particles of the binder resin added supplementally in order to form the shell layers do not successfully adhere to the core agglomerated particles to remain as floating particles standing unreacted or come to be liberated from the core agglomerated particles in the course of the fusion. This has been found as a result of our studies. If such unreacted particles remain, it is difficult for shells to adhere uniformly to core particles, making it difficult to achieve both the desired low-temperature fixing performance and blocking resistance. This phenomenon may become remarkable especially where, between binder resin particles making up the core particles and binder resin particles making up the shell layers, the binder resin particles making up the shell layers are larger in critical agglomeration concentration, i.e., where the binder resin particles making up the shell layers are higher in dispersion stability and hence can more not easily come to agglomerate. As a specific example thereof, a case may be given in which the binder resin particles making up the core particles have a carboxyl group as an acidic group and the binder resin particles making up the shell layers have a sulfonic acid group as an acidic group.